The present invention generally relates to signal pickup devices in rotary recording medium reproducing apparatuses, and more particularly to a signal pickup device constructed to enable a reproducing element which reproduces recorded signals from a rotary recording medium, to finely follow and trace the rotary recording medium regardless of the surface oscillation of the rotary recording medium introduced upon rotation of the rotary recording medium.
Generally, surface oscillation is introduced when a rotary recording medium (hereinafter referred to as disc) is rotated at high speed, and the signal recording surface of the disc undergoes microscopic oscillation. This oscillation includes low frequency components and their harmonic frequency components due to the surface oscillation caused by the imperfect recording surface of the disc, that is, since the recording surface of the disc is not completely flat, and irregularities exist on the recording surface. Furthermore, the above oscillation also includes mid-range frequency components due to vibration in the reproducing apparatus and the like, and low frequency repeated component of a signal recorded as pits formed intermittently on the disc surface (the horizontal synchronizing signal, vertical synchronizing signal, and tracking reference signal in a video signal, for example) and the like. Accordingly, the above oscillation extends in a frequency range from approximately 15 Hz to approximately 16 kHz.
The reproducing element which relatively scans the recording surface of the disc and reproduces the recorded signal, is required to finely trace and scan the recording surface of the disc regardless of the surface oscillation and vibration, and reproduce the recorded signal from the disc with desirable S/N (signal-to-noise) ratio.
Furthermore, even when pressure is applied to the reproducing element so that the reproducing element finely traces and scans the disc surface regardless of the surface oscillation and vibration of the disc, the amount of the above pressure which can be applied to the reproducing element is limited so as not to introduce scratches in either the reproducing element or the disc.
Moreover, the tracing limit of the reproducing element when the reproducing element is supported and pressed against the disc with a constant force, is determined by a constant acceleration defined by the equivalent mass at a tip end of a cantilever which supports the reproducing element, and the pressing force applied to the reproducing element. When the acceleration of the disc oscillation becomes larger than the above acceleration, the reproducing element cannot perform the tracing and scanning of the signal recording surface of the disc.
In a signal pickup apparatus comprising tracking control means for applying a force which applies stylus pressure on a reproducing stylus, and a force which displaces the reproducing stylus in a direction perpendicular with respect to the track longitudinal direction of the disc and performs the tracking control, to the rear end of a cantilever which supports the reproducing element at the tip end thereat, a tracking shift signal obtained from the reproduced signal is applied to tracking control means by use of a tracking servo loop. The above operation is performed in order to compensate for the distortion in the track and eccentricity introduced upon formation of the disc, of the forwarding movement error in an apparatus which subjects the reproducing element to perform a forwarding movement in the radial direction of the disc, and the like. Normally, when the above type of a tracking servo loop is an open servo loop, the tracking control signal is fed back having a gain of 70 dB, for example, with respect to the movement of the tip end of the cantilever, and designed so that the response extends to sufficiently high frequencies. The resonant frequency of the parasitic vibration due to the displacing movement of the cantilever which is included in this tracking servo loop, must be sufficiently higher than the frequency fc (1.5 kHz, for example) at which the above gain becomes zero.